1. Field of the Invention
The present invention relates to an improved process for preparing a thin metallic layer type magnetic recording medium by a tilt vapor deposition process.
2. Description of the Prior Art
Recently, high density recording of data has been remarkably required in development of magnetic recording media. The high density magnetic recording technology has been remarkably developed by improvements of a magnetic recording medium, a magnetic reproducing system, a running system, a magnetic head etc. Various studies are still continuing. Among these improvements, improvements of magnetic recording media are especially important as the fundamental matter. In the improvement, the required characteristics of the magnetic recording media are mainly an increase of coercive force, an increase of residual magnetic flux density, and a decrease of a thickness of a magnetic recording medium.
The conventional coating type magnetic recording media have been prepared by coating a magnetic paint comprising a magnetic powder and an organic binder on a substrate. Remarkable development for high density recording has been attained by using a metallic magnetic powder instead of iron oxide type magnetic powders as the magnetic powder. From a theoretical view-point, however, it is difficult to give a residual magnetic flux density of greater than 3,000-4,000 gauss because of the binder in such coating type magnetic recording media. Recently, metallic layer type magnetic recording media which do not contain an organic binder have been developed. The thin metallic layer type magnetic recording media have been prepared by forming a thin metallic layer on a substrate by forming metallic particles made of a metal or alloy of a iron group element by a vacuum evaporation, sputtering, ion-plating, ion beam evaporation or electrochemical method.
Among these methods, the vacuum evaporation method has been especially developed as the technology for preparing uniform, long magnetic recording media in an industrial scale, and the studies for this method has been in progress.
The following methods have been proposed to increase a coercive force and to improve squareness in the vacuum evaporation method.
(I) a tilt vapor deposition; PA1 (II) a deposition in a magnetic field; PA1 (III) a control by a selection of a material for a base; and PA1 (IV) a crystalline growth by a heat treatment.
The tilt vapor deposition method (I) is especially superior in view of the improvement of magnetic characteristics.
The tilt vapor deposition method utilizes a phenomenon of uniaxial magnetic anisotropy resulted by depositing metallic particles by flowing a metallic vapor at a tilt angle to a surface of a substrate as disclosed in Japanese Examined Patent Publication No. 19389/1966. The tilt vapor deposition phenomenon is remarkably complex and the reason for the magnetic anisotropy has not been found. It has been considered to result the phenomenon by self shade effect, an acicular growth of crystals and a magnetic distortion effect etc.
In the conventional process for preparing a magnetic recording medium by a typical tilt vapor deposition, a substrate is taken out to run the substrate through a cylindrical can to a wind-up shaft and a metallic vapor flow from a metal evaporation source is fed at a tilt angle to the substrate during the running of the substrate along a part of the peripheral surface of the cylindrical can in the vacuum tank.
FIG. 1 is a schematic view of the process. In the vacuum tank, a substrate-feeding shaft, a wind-up shaft and a cylindrical can X are placed to provide the running system for the substrate (1). A magnetic metal evaporation source O is placed at the bottom of the vacuum tank to feed metallic vapor flow to the facing part of the cylindrical can X. The metallic vapor flow is fed at a tilt angle .theta..sub.1 at the point A, and at a tilt angle .theta..sub.2 at the point B. Between the points A and B, the tilt angles are continuously varied in a range of .theta..sub.1 to .theta..sub.2. The magnetic anisotropy of the magnetic layer is depending upon the tilt angle .theta. and accordingly, .theta..sub.1 -.theta..sub.2 should be limited to certain ranges. The vapor deposited area is limited to the range between the point A and the point B. When the vapor deposited area is increased to the point C, the tilt angle .theta..sub.3 for the point C is remarkably different from .theta..sub.1, whereby the magnetic characteristic of the deposited metallic magnetic layer is inferior. Thus, the conventional tilt vapor deposition process has the disadvantage of low productivity.